The present invention relates to processes for producing potassium sulfate from potash and a source of sodium sulfate, such as anhydrous sodium sulfate or bloedite.
The overall reaction for producing potassium sulfate from sodium sulfate and potash can be described as: EQU Na.sub.2 SO.sub.4 +2KCl=K.sub.2 SO.sub.4 +2NaCl
In water, however, the reaction is subject to the thermodynamic constraints of the Na.sub.2 SO.sub.4 /2NaCl/K.sub.2 SO.sub.4 KCl/H.sub.2 O system. The data for the system are most conveniently represented on a Janeeke phase diagram (FIG. 1). The relevant invariant points, which are referred to in the subsequent description, are:
______________________________________ invariant point (a) solution in equilibrium with Na.sub.2 SO.sub.4, NaCl and glaserite (K.sub.3 Na(SO.sub.4).sub.2); invariant point (b) solution in equilibrium with KCl, NaCl and glaserite; invariant point (c) solution in equilibrium with KCl, K.sub.2 SO.sub.4 and glaserite. ______________________________________
The compositions of the invariant points (a), (b) and (c) at 25.degree. C. are as follows:
______________________________________ INVARIANT K.sub.2.sup.+2 Cl.sub.2.sup.-2 H.sub.2 O POINT (mole %) (mole %) mole/mole salts ______________________________________ (a) 14.6 79.2 14.9 (b) 29.3 93.7 14.7 (c) 68.6 94.5 19.5 ______________________________________
From the phase diagram (FIG. 1), it is evident that for any feed mixture of potash, sodium sulfate, and water, pure sodium chloride cannot be removed as a by-product. In addition, it is apparent that a reasonable potassium conversion can only be achieved in a two-stage reaction through the intermediate product glaserite. The two-stage reaction is illustrated schematically in FIG. 2 and includes the following stages:
Stage 1: Production of glaserite from sodium sulfate, potash, and Stage 2 liquor;
Stage 2: Production of potassium sulfate from potash, water, and glaserite from Stage 1.
The glaserite produced in Stage 1 is separated from the mother liquor in a suitable solid/liquid separator and introduced into Stage 2. Potash and water are introduced along with the glaserite and any unreacted potash from Stage 1. The potash and glaserite solids dissolve, generating a supersaturation solely with respect to potassium sulfate, such that potassium sulfate is selectively precipitated. The maximum conversion is obtained when the mother liquor approaches the KCl/K.sub.2 SO.sub.4 /glaserite/H.sub.2 O invariant point. The potassium sulfate slurry is separated and dried. The mother liquor removed from the reactor is returned to Stage 1.
The separated liquor from the glaserite contains substantial quantities of dissolved potassium and sulfate, which generally warrants a recovery operation. Currently known processes use the two-stage configuration, but differ in the scheme used to retrieve the potassium and sulfate values.
There are numerous problems associated with the solid/liquid separation of glaserite which is required in all known processes. Large glaserite particles filter relatively well but require a long residence time in the glaserite formation stage (Stage 1) and an even longer residence time in Stage 2, due to the slow dissolution kinetics of glaserite.
Small glaserite particles filter poorly. Moreover, the amount of adhering mother liquor is greatly increased with decreasing particle size. The mother liquor is rich in sodium (65-86 mole %) and in chloride (75-95 mole %), both of which reduce product quality. While some of the sodium introduced to Stage 2 can leave with the potassium sulfate product as glaserite, provided that the potassium content of the product satisfies the product specification, the rest must be dissolved in the effluent liquor.
Another source of "sodium-poisoning" is the potash feed. Agricultural-grade potash typically contains 3-4% NaCl, which must also be removed in the Stage 2 effluent liquor. Since at the optimum theoretical operating point (the KCl/K.sub.2 SO.sub.4 /glaserite/H.sub.2 invariant point), the solution contains .about.71% H.sub.2 O and under 3% sodium, about 25 kg of excess water must be added in Stage 2 to remove each additional kg of sodium introduced.
Sodium poisoning decreases the potassium conversion of the reaction train, and increases the feed water requirements, as can be seen from FIG. 3. Hence, the evaporation load in the recovery stage is increased, as are the equipment and energy costs. Energy costs are further increased because of additional heating and cooling costs for the enlarged recycle streams.
Moreover, the Stage 2 mother liquor adhering to the potassium sulfate crystals is a major source of sodium and chloride in the product, and can necessitate additional and costly process stages such as repulping or thorough washing.
Thus, there is a widely recognized need for, and it would be highly advantageous to have, a way of producing potassium sulfate from sodium sulfate which would be more efficient and more economical than heretofore known.